Transdifferentiation of Adult Frog Iris in Retina or Lens by Exogeneous Influences

Regenerative capacity of retinal cells and the maintenance of their differentiation

Mechanisms underlying cell type stability and the capacity of retinal cells for transdifferentiation are discussed. It is shown that cells of amphibian pigmented epithelium can be transformed into retina or lens cells depending on the inducing cell type: the influence of retina enables them to be transformed into retina, the influence of lens epithelium, to lens cells (lentoids or lenses). This led to an attempt to discover the molecular character of cell action by means of transfilter induction in early gastrula ectoderm of Xenopus laevis. The results show that the induced cell types correspond to the main inducing cell type, around which a range of neighbouring cell types is produced; this has been shown for five different cell types. The inducing factors involved seem to show qualitative differences. It is probable that they play a stabilizing role in the maintenance of the differentiated state of tissues, since temporary dissociation into cells leads eye tissues to transdifferentiate into other types. Such molecular factors can play a significant role in the maintenance of the type of differentiation and also in conversion into other cell types. These mechanisms of maintenance are not restricted to interactions between molecules and cells, since membranes on the surface of the retina and pigmented epithelium contribute to their shaping and consequently to the stability of the cell type.

The cellular and molecular bases of vertebrate lens regeneration

Lens regeneration takes place in some vertebrates through processes of cellular dedifferentiation and transdifferentiation, processes by which certain differentiated cell types can give rise to others. This review describes the principal forms of lens regeneration that occur in vivo as well as related in vitro systems of transdifferentiation. Classic experimental studies are reviewed that define the tissue interactions that trigger these events in vivo. Recent molecular analyses have begun to identify the genes associated with these processes. These latter studies generally reveal tremendous similarities between embryonic lens development and lens regeneration. Different models are proposed to describe basic molecular pathways that define the processes of lens regeneration and transdifferentiation. Finally, studies are discussed suggesting that fibroblast growth factors play key roles in supporting the process of lens regeneration. Retinoids, such as retinoic acid, may also play important roles in this process.

Transdifferentiation of adult human pigment epithelium into retinal cells by transfection with an activated H-ras proto-oncogene

The identification of homologs to viral oncogenes in normal cells coupled with development of techniques for DNA transfer into cells offers a powerful approach to dissect the processes associated with differentiation-specific oncogenes. We have derived cell lines by transfection of viral DNAs and proto-oncogenes into primary retinal pigment epithelial (RPE) cells. Establishment of cell lines was successfully achieved with the SV40 large T-antigen gene activated form of Harvey (H)-ras proto-oncogene, c-myc, and adenovirus E1A. The cell lines derived using the H-ras oncogene appeared to contain cells with a neuronal phenotype. This feature was not observed in cell lines established with the other oncogenes. Characteristically, H-ras-transfected cells all exhibited features associated with neurons around 10-14 passages. The transdifferentiated cells were biochemically characterized and found to express neuronal markers, such as neurofilament protein and neuron-specific enolases. The specific neuronal changes were restricted to only two primary cultures of RPE derived from carcinoma donors. Although transdifferentiation of pigmented cells of iris, or the retina, into the lens has been demonstrated, our studies presented in this report provide evidence that RPE cells from adults can transdifferentiate into neurons under the influence of a specific oncogene. To the best of our knowledge, this is the first report on transdifferentiation of adult human pigment epithelium into a neuronal cell type.

Transdifferentiation of ocular tissues in larval Xenopus laevis

Transdifferentiation phenomena offer a useful opportunity to study experimentally the mechanisms on which cell phenotypic stability depends. The capacities of vertebrate eye tissues to reprogram cell differentiation are well known in avian and mammalian embryos, and in larval and adult newt. From research into the capacity of anuran eye tissues to reprogram differentiation into a new pathway, considerable data have accumulated concerning the transdifferentiative capacities of eye tissues in larval Xenopus laevis. This work reviews the data concerning the transdifferentiative phenomena of eye tissues in that species and, based on these, aims to establish the extent of our knowledge about the mechanism controlling these processes. In larval Xenopus laevis the outer cornea can regenerate a lens by a lens-transdifferentiation process triggered and substained by a factor(s), probably of a protein nature, produced by the neural retina. In a normal eye phenotypic stability of the outer cornea is guaranteed by the presence of the inner cornea and lens, which prevent the spread of retinal factor(s). The stimulus for lens transdifferentiation of the outer cornea can be supplied by other tissues as well, but this capacity is not widely distributed. The iris and retinal pigmented epithelium can transdifferentiate into neural retina if isolated from the surrounding tissues and implanted in the vitreous chamber. As for lens transdifferentiation of the outer cornea, retinal transdifferentiation of the iris can be stimulated by certain nonocular tissues as well.

Transdifferentiation of pigmented epithelium induced by the influence of lens epithelium in frogs

The influence of lens epithelium (LE) of adult frogs on the character of transdifferentiation of retinal pigmented epithelium (RPE) of adult frogs and tadpoles of Rana temporaria has been studied. After a period of intense proliferation RPE cultured in vivo in contact with LE in the tadpole orbit almost exclusively transforms into retina. RPE precultivated in vitro in contact with LE for three days in protein-free medium does not manifest cell divisions and mostly transdifferentiates into lentoids. The problem of the relative significance of inducing determinants and the role of activation or inhibition of proliferation in transdifferentiation is discussed.